Root canals are needed for several reasons
You have a large cavity that is invading the internal blood vessels and nerve of the tooth, and there is a bacterial infection of those tissues. This may or may not hurt, and is diagnosed by looking at x-rays.
You have a tooth that is very painful because the nerve is dysfunctional–can be due to trauma, deep decay (see above), or, in some rare cases, there may not be a discernable cause.
You have a tooth that is “dead,” i.e. the nerves and blood vessels inside the tooth are dead and necrotic, literally rotting away inside the tooth. Tooth may or may not be hurting. This dead material needs to be removed via root canal procedure, because if we leave it behind, you may develop a massive infection at some point which can be life threatening.